I believe the fencing term is "corps-a-corps" or something like that, right?
Question: Couldn't the 20th-level fighter use Bull Rush or Overrun to escape?
It sounds like the "grappling" effect you want is the one that limits both grapplers to attacking with light weapons only. How's this (just off the top of my head):
Corps a corps
You are skilled in closing inside the enemy's guard during melee.
Prerequisite: BAB +3, Dex 13, Combat Expertise
Benefit: You may end your move in an enemy's square by succeeding in an opposed BAB Check. You gain a +4 bonus on this check. If you fail, you stop before entering the opponent's square and provoke an attack of opportunity. If you succeed, you are "corps-a-corps" and share the square with your opponent. Moving out of the shared square does not provoke an attack of opportunity from the other person involved, but does require a Move action (not a 5-foot step). This movement may draw AoO from others. While corps-a-corps you are both at -4 to AC and attack rolls. In addition, enemies sharing a square may only attack one another with light or natural weapons. Grapples are initiated and resolved normally for corps-a-corps characters.
Normal: You many not end your movement in another character's space.